(a) Fields of the Invention
The present invention relates to photomasks and methods for forming a pattern using the photomask. In particular, the present invention relates to methods for forming, in the same step, a via feature and a plurality of slit features intersecting or joining each other.
(b) Description of Related Art
Line features or slit features formed on a semiconductor substrate are required to be formed faithfully based on design patterns of a semiconductor circuit. Thus, if a resist pattern having a bent shape that is difficult to form faithfully based on a design pattern is formed on a semiconductor substrate, a photomask is used which is provided with assist light-shielding portions or assist light-transmitting portions.
As an example of such a technique, a proposal is made if a conventional pattern formation method disclosed in Japanese Unexamined Patent Publication No. 2002-116459. Hereinafter, the conventional pattern formation method using a photomask provided with assist light-shielding portions or assist light-transmitting portions will be described with reference to FIGS. 7A, 7B, 8A, 8B, 9A, and 9B.
FIGS. 7A and 7B are plan views showing exemplary design patterns. FIGS. 8A and 8B are plan views showing conventional photoresist patterns. FIGS. 9A and 9B are plan views showing conventional reticle patterns with assist features.
Referring to FIGS. 7A and 7B, when a semiconductor device or the like is designed, a line feature 1a as shown in FIG. 7A or a slit feature 2a as shown in FIG. 7B is employed in the design stage for the purpose of configuring circuitry in the device. If the line feature 1a shown in FIG. 7A is formed, a mask for exposure is employed in which a region occupied by the line feature 1a is used as a light-shielding portion during exposure and in which a region 1b surrounding the line feature 1a is used as a light-transmitting portion during exposure. If the slit feature 2a shown in FIG. 7B is formed, a mask for exposure is employed in which a region occupied by the slit feature 2a is used as a light-transmitting portion during exposure and in which a region 2b surrounding the line feature 2a is used as a light-shielding portion during exposure.
One means for forming a line feature or a slit feature on a semiconductor substrate based on the design pattern shown in FIG. 7A or 7B is means by which a photoresist film is patterned.
As an example, the following discusses the case where a positive photoresist film is patterned. If use is made of a photomask on which the above line feature 1a shown in FIG. 7A is transferred, the line feature 5a as shown in FIG. 8A is formed in the film. If use is made of a photomask on which the above slit feature 2a shown in FIG. 7B is transferred, the slit feature 6a shown in FIG. 8B is formed in the film.
To be more specific, as shown in FIG. 8A, the line feature 5a is formed of a remaining portion of the photoresist film and surrounded with the region 5b where the photoresist film has been removed. A dotted line 5c indicates the contour of the line feature 1a shown in FIG. 7A. As is obvious from the comparison between the line feature 5a and the dotted line 5c, an area located within the region surrounded with the dotted line 5c and not formed with the photoresist film or an area located without that region and formed with the photoresist film exists in the vicinity of each bent portion of the line feature 5a. 
On the other hands, as shown in FIG. 8B, the slit feature 6a is formed of a region in which the photoresist film has been removed and which is surrounded with a remaining portion 6b of the photoresist film. A dotted line 6c indicates the contour of the slit feature 2a shown in FIG. 7B. Like the previous case, as is obvious from the comparison between the slit feature 2a and the dotted line 6c, an area located within the region surrounded with the dotted line 6c and formed with the photoresist film or an area located without that region and not formed with a photoresist film exists in the vicinity of each bent portion of the slit feature 6a. 
Ideally, the line and slit features 5a and 6a shown in FIGS. 8A and 8B should be formed in the same shapes as the line feature 1a (see FIG. 7A) and the slit feature 2a (see FIG. 7B) indicated by the dotted lines 5c and 6c, respectively. However, actually, as shown above, the bent portions of the line feature 5a and the slit feature 6a are not formed in the same shapes as those of the line feature 1a (see FIG. 7A) and the slit feature 2a (see FIG. 7B).
As mentioned above, the problem that the design pattern and the photoresist pattern formed based on the designed pattern differs in shape becomes more severe as the pattern is finer.
To overcome this problem, in the conventional pattern formation method, photomasks as shown in FIGS. 9A and 9B having patterns with assist features 7c, 7d, 8c, and 8d additionally attached to the comers thereof are employed in order for the photoresist pattern formed based on the designed pattern to have the same shape as the design pattern.
To be more specific, in the case of a reticle pattern shown in FIG. 9A, the corners of a light-shielding portion 7a formed in a light-transmitting portion 7b for transmitting light during exposure are provided with assist features 7c serving as light-shielding portions during exposure and an assist feature 7d serving as a light-transmitting portion during exposure. Likewise, in the case of a reticle pattern shown in FIG. 9B, the corners of a light-transmitting portion 8a formed in a light-shielding portion 8b for shielding light during exposure are provided with assist features 8c serving as light-transmitting portions during exposure and an assist feature 8d serving as a light-shielding portion during exposure. When the corners of the pattern are provided with the assist features 7c, 7d, 8c and 8d, this reduces the phenomenon that light diffracts around or does not come into the corners of the pattern during exposure. Therefore, a photoresist pattern having almost the same shape as the design pattern can be formed.
However, if, in the step of forming via features, via features as shown in FIG. 10A as well as a plurality of slit features as shown in FIG. 10B intersecting or joining one another and having widths within a predetermined size are simultaneously formed, the light intensity required for the formation of these features inevitably becomes extremely high throughout the entire photoresist film for successful formation of the via features. Therefore, if pattern formation is carried out in the same step using a photomask as shown in FIG. 10A employed for forming via features and composed of light-transmitting portions 12a and a light-shielding portion 12b and a photomask as shown in FIG. 10B employed for forming a plurality of slit features intersecting or joining one another and composed of a light-transmitting portion 12c and a light-shielding portion 12d, slit features 13c formed in a photoresist 13d shown in FIG. 11B have greater widths than via features 13a formed in a photoresist 13b shown in FIG. 11A even though the length of each side of the light-transmitting portion 12a shown in FIG. 10A is equal to the width of the light-transmitting portion 12c shown in FIG. 10B. Moreover, the reason why the regions of the slit features 13c in FIG. 11B located around the intersecting or joining point are larger than the regions of the light-transmitting portion 12c in FIG. 10B located around the intersecting or joining point is that light diffraction strengthens around the intersecting or joining point of the slit features 13c. As is apparent from the above, it is extremely difficult to form, in the same step, the via features and the plurality of slit features intersecting or joining one another and having widths within a predetermined size.